1. Field of the Invention
The present invention relates to the noncatalytic reduction of NO.sub.x and particulates in engine exhaust, and more particularly to systems for decomposing NO.sub.x to N.sub.2 and O.sub.2 and particulates to CO.sub.2 in oxygen-rich environments.
2. Description of Related Art
The control of NO.sub.x emissions from vehicles is a worldwide environmental problem. Gasoline engine vehicles can use newly developed three-way catalysts to control such emissions, because their exhaust gases lack oxygen. But so-called "lean-burn" gas engines, and diesel engines too, have so much oxygen in their exhausts that conventional catalytic systems are effectively disabled. Lean-burn, high air-to-fuel ratio, engines are certain to become more important in meeting the mandated fuel economy requirements of next-generation vehicles. Fuel economy is improved since operating an engine stoichiometrically lean improves the combustion efficiency and power output. But excessive oxygen in lean-burn engine exhausts can inhibit NO.sub.x removal in conventional three-way catalytic converters. An effective and durable catalyst for controlling NO.sub.x emissions under net oxidizing conditions is also critical for diesel engines.
Catalysts that have the activity, durability, and temperature window required to effectively remove NO.sub.x from the exhaust of lean-burn engines are unknown. Prior art lean-NO.sub.x catalysts are hydrothermally unstable. A noticeable loss of activity occurs after relatively little use, and even such catalysts only operate over very limited temperature ranges. Conventional catalysts are therefore inadequate for lean-burn operation and ordinary driving conditions.
Catalysts that can effectively decompose NO.sub.x to N.sub.2 and O.sub.2 in oxygen-rich environments have not yet been developed, although it is a subject of considerable research. But see, U.S. Pat. No. 5,208,205, issued May 4, 1993, to Subramanian, et al. An alternative is to use catalysts that selectively reduce NO.sub.x in the presence of a co-reductant, e.g., selective catalytic reduction (SCR) using ammonia as a co-reductant.
Using co-existing hydrocarbons in the exhaust of mobile lean-burn gasoline engines as a co-reductant is a more practical, cost-effective, and environmentally sound approach. The search for effective and durable SCR catalysts that work with hydrocarbon co-reductants in oxygen-rich environments is a high-priority issue in emissions control and the subject of intense investigations by automobile and catalyst companies, and universities, throughout the world.
SCR catalysts that selectively promote the reduction of NO.sub.x under oxygen-rich conditions in the presence of hydrocarbons are known as lean-NO.sub.x catalysts. More than fifty such SCR catalysts are conventionally known to exist. These include a wide assortment of catalysts, reductants, and conditions. Unfortunately, just solving the problem of catalyst activity in an oxygen-rich environment is not enough for practical applications. Like most heterogeneous catalytic processes, the SCR process is susceptible to chemical and/or thermal deactivation. Many lean-NO.sub.x catalysts are too susceptible to water vapor and high temperatures. As an example, the Cu-zeolite catalysts deactivate irreversibly if a certain temperature is exceeded. The deactivation is accelerated by the presence of water vapor in the stream. In addition, water vapor suppresses the NO reduction activity even at lower temperatures.
The problems encountered in lean-NO.sub.x catalysts include lessened activity of the catalyst in the presence of excessive amounts of oxygen, reduced durability of the catalyst in the presence of water, sulfur, and high temperature exposure, and narrow temperature windows in which the catalyst is active. Practical lean-NO.sub.x catalysts must overcome all three problems simultaneously before they can be considered for commercial use.
Lean-burn engine exhausts have an excessive amount of oxygen that renders conventional three-way catalytic converters useless for NO.sub.x removal. The excess oxygen adsorbs preferentially on the precious metal, e.g., Pt, Rh, and Pd, surfaces in the catalyst, and inhibits a chemical reduction of NO.sub.x to N.sub.2 and O.sub.2. A wide variety of catalysts and reductants are known to promote lean-NO.sub.x catalysis, however, all such catalysts have proven to be susceptible to chemical and/or thermal deactivation. Another major hurdle for commercialization of the current lean-NO.sub.x catalysts is the lack of durability in catalysts to the effects of high-temperature water vapor, which is always present in engine exhaust. Conventional lean-NO.sub.x catalysts are hydrothermally unstable and lose activity after only a short operation time.
Some gasoline can contain up to 1200 ppm of organo-sulfur compounds. These convert to SO.sub.2 and SO.sub.3 during combustion. Such SO.sub.2 will adsorb onto the precious metal sites at temperatures below 300.degree. C. and thereby inhibits the catalytic conversions of CO, C.sub.x H.sub.y (hydrocarbons) and NO.sub.x. At higher temperatures with an Al.sub.2 O.sub.3 catalyst carrier, SO.sub.2 is converted to SO.sub.3 to form a large-volume, low-density material, Al.sub.2 (SO.sub.4).sub.3, that alters the catalyst surface area and leads to deactivation. In the prior art, the only solution to this problem offered has been to use fuels with low sulfur contents.
Another major source of catalyst deactivation is high temperature exposure. This is especially true in automobile catalysts where temperatures close to 1000.degree. C. can exist. The high-temperatures attack both the catalyst precious metal and the catalyst carrier, e.g., gamma alumina (.gamma.-Al.sub.2 O.sub.3). Three-way catalysts are comprised of about 0.1 to 0.15 percent precious metals on a .gamma.-Al.sub.2 O.sub.3 wash coat, and use La.sub.2 O.sub.3 and/or BaO for a thermally-stable, high surface area .gamma.-Al.sub.2 O.sub.3. Even though the precious metals in prior art catalysts were initially well dispersed on the .gamma.-Al.sub.2 O.sub.3 carrier, they were subject to significant sintering when exposed to high temperatures. This problem, in turn, led to the incorporation of certain rare earth oxides such as CeO.sub.2 to minimize the sintering rates of such precious metals.
Because of the remarkable success that has been achieved in the use of modifiers for improving the durability of the modern catalytic converters, this same approach is being used in the attempt to improve the durability of lean-NO.sub.x catalysts. Much effort has therefore been devoted to the use of modifiers to improve the stability of lean-NO.sub.x catalysts in the simultaneous presence of water, SO.sub.2, and high temperature exposure. However, the results are still far from being satisfactory.
Another catalyst technology for NO.sub.x removal involves "lean NO.sub.x trap" catalysis. As with SCR lean-NO.sub.x catalysts, the lean-NO.sub.x trap technology can involve the catalytic oxidation of NO to NO.sub.2 by catalytic metal components effective for such oxidation, such as precious metals; however, in the lean NO.sub.x trap, the formation of NO.sub.2 is followed by the formation of a nitrate when the NO.sub.2 is adsorbed onto the catalyst surface. The NO.sub.2 is thus "trapped", i.e., stored, on the catalyst surface in the nitrate form and subsequently decomposed by periodically operating the system under stoiciometrically fuel-rich combustion conditions that effect a reduction of the released NO.sub.x to N.sub.2.
Both lean-NO.sub.x SCR and lean-NO.sub.x -trap catalysts, i.e., NO.sub.x reduction catalysts, have been limited to use for low sulfur fuels because catalysts that are active for converting NO to NO.sub.2 are also active in converting SO.sub.2 to SO.sub.3. Both lean NO.sub.x SCR and trap catalysts have shown serious deactivation in the presence of SO.sub.x because, under oxygen-rich conditions, SO.sub.x adsorbs more strongly on NO.sub.2 adsorption sites than NO.sub.2, and the adsorbed SO.sub.x does not desorb altogether even under fuel-rich conditions. Such presence of SO.sub.3 leads to the formation of sulfuric acid and sulfates that increase the particulates in the exhaust and poison the active sites on the catalyst. Attempts with limited success to solve such a problem have encompassed, for example, Nakatsuji et al. describing the use of selective SO.sub.x adsorbents upstream of lean NO.sub.x trap adsorbents.
Furthermore, catalytic oxidation of NO to NO.sub.2 is limited in its temperature range. Oxidation of NO to NO.sub.2 by a conventional Pt-based catalyst maximizes at about 250.degree. C. and loses its efficiency below about 100 degrees and above about 400 degrees. Thus, the search continues in the development of systems that improve lean NO.sub.x trap technology with respect to temperature and sulfur considerations.
A technology for the removal of carbon-containing particulates from lean-burn engine exhausts (particularly diesel exhausts) involves trapping such particulates--commonly called the "particulate trap." Particulate traps based upon interception, impaction and/or diffusion collections methods have been shown to significantly reduce carbon particulate emissions from diesel or lean-burn engine exhausts. Such collection methods have been combined with particulate disposal methods based upon electrical, mechanical and/or chemical techniques to achieve complete particulate trap systems. However, current particulate trap systems are expensive and unreliable.
The U.S. Federal Test Procedure for cold starting gasoline fueled vehicles presents a big challenge for lean-NO.sub.x catalysts due to the low-temperature operation involved. Diesel passenger car applications are similarly challenged by the driving cycle that simulates slow-moving traffic. Both tests require reductions of CO, hydrocarbons, and NO.sub.x at temperatures below 200.degree. C. when located in the under-floor position. Current EPA standards for particulate emission limits are approximately 0.1 g/Bhp-hr while NO.sub.x requirements are less than about 4 g/Bhp-hr. Future particulate and NO.sub.x emission standards are even more stringent. Modifications of existing catalyst oxidation technology are successfully being used to address the problem of CO and hydrocarbon emissions, but no cost-effective solution exists for NO.sub.x and particulate emissions.